This invention relates to a process for fabricating a humidity-sensing element, and more specifically to a process for fabricating a humidity-sensing element of the electrical resistance type using ceramics based on ZrO.sub.2, ZrO.sub.2 +Y.sub.2 O.sub.3 and Y.sub.2 O.sub.3. The humidity-sensing element according to this invention meets all the characteristic requirements for an element of this type, including low electrical resistivity, electrical resistance-humidity linearity, high sensitivity, and small variation with time. Moreover, it shows stable characteristics with very slight deviation of the electrical resistance-humidity characteristic curve in high and low humidity atmospheres, as well as in the ordinary humidity range.
Humidity-sensing elements in recent years have found widespread applications. With home appliances, they are often used in controlling the cooking by microwave ovens, determining the drying degrees of laundry in driers, monitoring humidity under control by air conditioners, and in detecting dewing of the cylinders in video taperecorders. For industrial applications, the elements are in widespread use for humidity control in the manufacture of various electronic parts. Among other fields in which they are finding use are air conditioning in agricultural greenhouses and prevention of dewing on rear-window defoggers of automobiles. For automated systems of food preparation, air conditioning, drying, and other operations, control of humidity, as well as of temperature, is now indispensable. Thus, there is a need for the development of humidity-sensing elements capable of functioning with higher reliability than heretofore.
In order to meet this requirement, humidity-sensing elements that utilize changes in electric resistance have come into use. They detect changes in the humidity content of a given object as changes in its electric resistance. The humidity-sensitive material so far proposed for sensor applications vary widely, from electrolytic substances typified by lithium chloride to organic polymers and ceramics. The humidity-sensing element of the resistance type is basically required to offer a low electrical resistance, good linearity of the resistance-humidity characteristic, proper operating range, and resistance to deterioration in the service environments. Ceramics have recently attracted growing attention as materials for humidity-sensing use, generally satisfying all the foregoing requirements.
As to the fabrication of the humidity-sensing elements, two types have been proposed. One is a bulk type in which a pair of electrodes is disposed on opposite sides of a ceramic sintered body. The other is a thick film type fabricated by forming an electrode layer on at least one side of a ceramic substrate, applying a mixture of the powder of the above ceramic substance with a binder to the electrode layer, and drying and sintering the coat to form a solid humidity-sensing part.
The literature on ceramic humidity-sensing elements includes the following:
Japanese Patent Application
Public Disclosure Nos. 152105/1982, 47703/1984, 166701/1983, 86447/1983. PA1 (A) Appropriately low electrical resistance (The greater the current the better the sensitivity.) PA1 (B) Good resistance-humidity characteristic linearity PA1 (C) High sensitivity PA1 (D) High precision PA1 (E) High stability PA1 (F) Adequate reproducibility PA1 (1) Y.sub.2 O.sub.3, PA1 (2) Y.sub.2 O.sub.3 +ZrO.sub.2 (0.01-99.00%), PA1 (3) Y.sub.2 O.sub.3 +at least one of (CaO, MgO, BaO, TiO.sub.2, Ta.sub.2 O.sub.3, Nb.sub.2 O.sub.3, and V.sub.2 O.sub.5) (0.01-99.00%), and PA1 (4) ZrO.sub.2 +at least one of (CaO, MgO, BaO, TiO.sub.2, Ta.sub.2 O.sub.3, Nb.sub.2 O.sub.3, and V.sub.2 O.sub.5) (0.01-99.00%). PA1 (1) Porosity: 34-85% PA1 (2) Pore size: 0.01-3 .mu.m PA1 (3) Thickness: 20-200 .mu.m